Colossus
Download a Java version of Avalon Hill's Titan boardgame. Currently 'hotseat' play only.
colossus.sourceforge.net Details

CyberBoard
Board gaming system for playing board games by electronic mail. Graphically design the various parts of a board game on your computer, the players make their moves and exchange recorded versions of the moves with their opponents. Free download. For Windows.
cyberboard.brainiac.com Details

GUB: the Generic Universal Boardgame
A Java 2 application that allows players to play almost any type of board game on their computer, using a simple text file with game piece descriptions, a GIF or JPG copy of a game board, and an optional set of graphics for the game pieces. Several game piece sets are distributed here, and others may be created by other people and distributed from elsewhere.
ludo.iwarp.com Details

JET
Computer aide and PBEM tool for Europa series of WWII games. Supports many basic rules of the game system. Open source, Windows executable.
www.microteam.fi Details

Strategic Map Simulator (SMS)
A wargame, RPG, and PBeM utility for playing board games over long distances. Free download for Windows.
www.flyingzebra.com Details

Warhorse Simulations
http://www.warhorsesim.com/acts/
Details